Precise method for controlling thickness of a coating

ABSTRACT

AS A COATING COMPOSITION IS DEPOSITED ON A TRAVEL WEB AS THE WEB PASSES OVER A BACK UP ROLLER DISTURBANCES SUCH AS VARIATION IN THE RATE OF WEB FEED, THE THICKNESS OF THE WEB AND/OR VIBRATIONS ARE TRANSLATED INTO AN ELECTRICAL SIGNAL WHICH IS THEN PASSED THROUGH A POLARITY CONVERTER, A PHASE SHIFTER AND AN AMPLIGER. THE RESULTING SIGNAL IS APPLIED WITH A SUITABLE BIAS VOLTAGE BETWEEN THE COATING COMPOSITION AND A SUITABLE ELECTRODE ON THE OPPOSITE SIDE OF THE WEB AT THE POINT OF APPLICATION.

Jan. 16, 1973 AZUTAKA os m ETAL 3,711,312

LLING THICKNESS PRE E METHOD FOR CONTRO 0F OATI Original F'i June 1969 FIG. I

INVENTORS. I2 w 8 TAKA YOSHIDA M ORU ISHIWATA f flu, in; M

United States Patent US. Cl. 117-34 Claims ABSTRACT OF THE DISCLOSURE As a coating composition is deposited on a traveling web as the web passes over a back up roller disturbances such as variations in the rate of Web feed, the thickness of the web and/or vibrations are translated into an electrical signal which is then passed through a polarity converter, a phase shifter and an amplifier. The resulting signal is applied with a suitable bias voltage between the coating composition and a suitable electrode on the opposite side of the web at the point of application.

This is a continuation of application S.N. 831,875 filed June 10, 1969, now abandoned.

This invention relates to a method of controlling a change of a coating thickness of a liquid layer or layers successively coated on a web, such as paper, plastics film or the like.

This method is suited especially for coating photographic light sensitive emulsions, such as a gelatino-silver halide solution, on a web surface and is also applicable for multi-layer coating.

Among the various methods which have been proposed or used for coating liquids onto webs is a doctor coating method and a bead coating method. These methods are used for coating photographic light-sensitive emulsions, such as a gelatino-silverhalide solution, but in these methods there occur changes of the coating thickness which are called Lateral defects and which are caused by the mechanical vibrations of the coating apparatus.

Therefore, an object of this invention is to provide a coating method capable of obtaining a uniform coated layer, irrespective of the mechanical vibrations of the coating apparatus.

Another object of this invention is to prevent the slight or minor change of the coating thickness, irrespective of the change of the web thickness, irregularity of the web speed and other external disturbances.

This invention is intended to make use of an electric signal applied between the coating composition and an electrode via the web for controlling or regulating the change of coating thickness.

More particularly, this invention is intended to apply an electrical signal which changes corresponding to the extent of the disturbance to a zone between the coating composition and an electrode via the web to regulate or prevent the change of the coating thickness due to the mechanical vibration of the coating apparatus, change of the web thickness, irregularity of the apparatus speed and other external disturbances.

ice

Now, this invention will be described with reference to the accompanying drawings, wherein,

FIG. 1 is a diagrammatic view, partly in cross section, of an apparatus for preventing slight changes of coating thickness from being caused by the mechanical vibration of the coating apparatus and FIG. 2 is similar to the view in FIG. 1 but for preventing slight changes of the coating thickness from being caused by the change of the web thickness.

In FIG. 1, 1 denotes a continuously moving web, such as a paper sheet or a film, 2 is a backing roller electrically insulated from its shaft by an insulating layer 3, 4 is a coating device for applying the coating composition 6 fed by a pump 5, 7 is a vibration meter which detects the mechanical vibration of the coating device 4 and converts said vibration into an electrical signal, 8 is a polarity converter which converts the polarity of the electric signal generated from the vibration meter 7, 9 is a phase shifter which deviates the phase of the polarity converted electric signal, 10 is a voltage amplifier for amplifying the electric signal treated by both the polarity converter 8 and the phase shifter 9 and for giving proper bias voltage to said electric signal. 11 is a terminal of the voltage amplifier 10 for providing a potential to the backing roller 2 via a lead wire and a slip ring 13, and 12 is another terminal of the voltage amplifier 10 for providing a potential to the coating solution or composition 6 in the coating device 4 via a lead wire and an electrode 14.

Where the web 1 is made of nonconductive material, the metallic backing roller 2 is not coated but where the web 1 is of the material which becomes conductive by impregnation with the coating composition 6, the surface of the metal roller must be coated with an insulating film.

Any type of coating device 4 which may be used in the doctor method, bead coating method, dip method or roller method is applicable to this process and the design of the coating device 4 may be of the hopper type, extrusion type or slide type. This process was advantageously achieved with the use of a coating device 4 suitable for the doctor method.

The pump 5 is generally a non-pulsating, constant feed pump and the coating composition 6 may beany one of several photographic light sensitive emulsions as a gelatino-silver halide solution, undercoating, solution, baryta solution, back lining solution, antihalation solution, middle layer coating solution and upper protecting layer coating solution.

The vibration meter 7 may be a piezo-electric type, a dynamic electric type, a dynamic magnetic type, or a capacitor type meter and the polarity converter 8, the phase shifter 9 and the amplifier 10 may be the electronic tube type or semiconductor type.

In the FIG. 2, 15 is a multiple doctor type multilayer coating device for applying successively pluralities of coating compositions on a web 1. The device 16 is a contact type or non-contact type detector for detecting the change of web thickness along the web moving direction and 17 is a delay circuit for the electric signal for correcting the time lag caused between the thickness change detector 16 and the doctor type multilayer coating device 15.

Although one of the terminals 11 of the voltage amplifier 10 is connected to the backing roller 2 via a lead wire and a slip ring 13 in both FIGS. 1 and 2, the terminal 11 may be connected also to an electrode plate or electrode of any other shape capable of generating an electrostatic field on the coating zone of the web, if the terminal is not to be brought into direct contact with the coating composition in the usual liquid state.

On the other hand, another terminal 12 of the voltage amplifier may be connected to the electrode 14 dipped directly into the coating composition reservoir as shown in FIG. 1 via the lead wire and also to the doctor ty e multi-layer coating device as shown in FIG. 2 if the coating device is made of conductive material.

If the backing roller 2 is not insulated by the insulating layer 3 but by other means adjacent the bearing thereof, the connection of the terminal 11 of the voltage amplifier 10 to the backing roller 2 does not necessarily require a slip ring 13 and the terminal may be connected directly to the bearing part or a rotary transformer can be utilized for the connection.

In the doctor coating process or the bead coating process, the mean amount of the coating composition to be applied to the web surface is determined by the feeding rate of the coating composition. The coating amount is to be kept at a theoretical constant value by selecting the feeding rate at predetermined value. However, the coating thickness is still changed slightly by the disturbance based on the mechanical vibration or" the coating part, the change of base thickness along the moving direction, or irregular speed of running web it the coating amount is observed at minor time unit, such as A sec, sec or ,4 sec.

For example, with the ordinary photographic light sensitive film producing apparatus having a mechanical vibration with 0-510.0}L amplitude and -100 H frequency at the coating part, a change of web thickness along the running direction with 110,L amplitude and 5-30 mm. pitch, and irregular web running speed with (LS-5.0% and 0.l50.0 H will cause a minor change of the coating thickness.

The minor change of the coating thickness varies considerably with the type of coating method and the coating conditions, and can be reduced to minimum level with the selection of a suitable coating method and coating conditions. In the case where the mean coating amount is decreased to reduce the drying load and to attain high speed coating, the regulation of the minor changes of the coating thickness becomes diflicult and lateral defects are formed along the web running direction based on the coating thickness irregularity.

In order to control a minor change of the coating thickness by any means other than the selection of the coating method and coating conditions, a method of changing the feeding rate of coating composition to prevent the change of coating thickness due to the disturbance was first proposed. But this method cannot respond quickly to the changes caused by the disturbances. Therefore, the prevention of a change of the coating thickness caused by disturbances is extremely diificult by any mechanical means.

This invention is intended to prevent the change of the coating thickness caused by disturbances by electric means and to minimize the change of coating thickness.

As shown in FIGS. 1 and 2, by applying voltage between 2 electrodes, one of which is the coating composition 6 from the coating device 4 or the doctor type multilayer coating device 15, and the other which is the backing roller 2 separated from the coating composition by the base web, an attractive force is caused between the coating composition 6 and the backing roller 2 by the electrostatic action. As the voltage applied between said 2 electrodes changes, the electrostatic attractive force between the 2 electrodes, i.e. the coating composition 6 and the backing roller 2, changes also and the amount of the coating composition 6 to be applied to the web 1, more particularly the coating thickness on the web, is changed.

This invention makes use of this phenomenon by following an inverse course; i.e. in thecase where the coating thickness is changed slightly by the vibration of the coating device 4, voltage should be applied to both electrodes to negate the vibration of the coating device. As the electrostatic action changes solely with the absolute value of the applied voltage, the electrostatic attractive force will not be converted into a repelling force irrespective of the reversal of the electrostatic field between two electrodes. Therefore, positive or negative bias voltage must be applied to any one of the electrodes to prevent the reversal of the electric field.

In order to prevent the change of the coating thickness in the FIG. 1, the mechanical vibration of the coating device is converted into an electric signal with several mv. amplitude by the vibration meter 7; the polarity converter 8 reverses the polarity of the electric signal from the. vibration meter '7 proportional to the vibration of the coating device; the phaseshifter 9 adjusts the .phase'jdeviation of the high voltage signal applied tothecoating device for controlling the change of coated thickness, and the voltage amplifier 10 amplifies the weak electric signal having an amplitude of several mv. intothe high voltage signal with an amplitude of several hundreds v. and at the same time imparts the positive or negative bias voltage of several hundreds v. v g

in the PEG. 2, the thickness change detector16 senses a change of the Web thickness, as an electric signalwith a frequency of from 10 to several hundreds H and an amplitude of several tens mv., in advance of the coating device; the polarity converter reverses the. polarity of the electric signal proportional tothe change of the, Web thickness and generated by the thickness change detector 16; the delay circuit 17 corrects the time delay of several tenths of a second between the thickness change detector 16 and the coating device; and the voltage amplifier -10 amplifies the weak electric signal of several tens mv. into a high voltage signal with several hundreds v. amplitude and at the same time imparts a positive or negative'bias voltage of several hundreds v. r

Furthermore, in the apparatus for controlling minor changes of the coated thickness as shown in the FIG. 1, it was ascertained by a test result using an opticaldensity irregularity detector, including photoelectric converting elements, that the Wave shape of the minor change of the coating thickness corresponds with that of the Weak electric signal obtained by the vibration meter 7 attached to the coating device 4 and therefore the controlling of the minor change of the coating thickness is made possible by this invention.

And also in the apparatus for controlling minor change of the coating thickness as shown in the FIG. 2, it was ascertained by a test result using an optical density irregularity detector having photoelectric converting elements that the wave shape of the minor change of the coating thickness agrees with that of the weak electric signal obtained by the thickness change detector 16 and therefore the controlling of the minor change of the coated layer thickness is made possible according to thi invention. I

An optical density irregularity detector can be applied immediately after coating to detect the minor change of coating thickness, while the obtained signal is amplified and fed back to control the phase shifter or the delay circuit, and whereby the automatic control of the minor change of the coating thickness is made possible; i

In the case where the thickness of the coating composition 6 coated on the running web 1 changes slightlydue to irregular web speed, it is possible to prevent or regulate the minor change of the coating thickness by detecting the irregular web speed with an angular acceleration meter, converting the detected value into an electric signal, passing said electric signal through the phase adjuster 9 or the delay circuit 17 and the voltage amplifier 10 and applying the obtained signal voltage for controlling the minor change of the coating thickness between the coating composition and the electrode located on the opposite side of the base web.

It is further possible according to this invention to prevent or regulate minor changes of the coating thickness by passing an electric signal which is obtained by a proper disturbance detector through the phase adjuster 9 or the delay circuit 17 and the voltage amplifier 10 and applying the resulting signal voltage for controlling minor changes of the coating thicknes between the coating composition and the electrode provided on the opposite side of the base web, in the case where the thickness of the coating composition 6 coated on the web 1 changes slightly.

Also, in the case where the thickness of the coating composition 6 coated on the web 1 changes slightly due to one or more disturbances selected from a group consisting of the mechanical vibration of the coating part, the change of the web thickness, the irregular web speed and other disturbance, it is possible to prevent or regulate the slight change of the coating thickness according to this invention by mixing several weak electric signals each of which is proportional to each of the disturbances and passing the mixed signal through the phase shifter 9 or the delay circuit 17 to have the proper phase deviation and the time delay, amplifying the mixed signal with the voltage amplifier 10, adding proper bias voltage to the said signal and applying the resulting signal between the coating composition and the opposite face of the base web.

It must be noted that the voltage applied to the coating device, in any one of the aforesaid cases, does not impart any effect on the mean coated amounts but relates exclusively to the minor changes of the coating thickness.

In any of these cases, the electric current flowing through the coating zone as a result of the voltage for controlling the coating thickness does not exceed several ten ma., and accordingly only consumes extremely small electric power. Because of the weak electric current, injurious effects, chemical or physical, cannot be imparted to the coating composition. By attaching a breaker with about 50 ma. capacity to this apparatus, this invention will be executed safely without danger of a fatal accident in case of improper handling.

When the voltage to be applied between the coating composition and the other electrode according to this method exceeds 1500 v., a phenomenon of electric discharge occurs between the coating composition and the electrode, thus causing a detrimental fog on the light sensitive emulsion layer. But, if the voltage does not exceed 1000 v. in the usual application, such an electric discharge is rarely caused.

Several advantages obtained by the application of this method are as follows:

(1) Uniform thickness of the coated layer is obtained irrespective of the mechanical vibration of the coating device, the change of the web thickness, the irregular web speed and other disturbances, and therefore high grade coated products, especially thin films coated with a uniform layer, are advantageously obtained.

(2) Saving costs for raw materials and plant-building is expected by the relatively mild requisition for the accuracies of the web and the apparatus.

(3) A wide selection of coating methods and coating conditions is made possible by the mild restrictions imposed on the latter for negating the effects of disturbances.

(4) A wide selection in the composition of the coating composition is also made possible for the same reasons as described above.

(5) As the exact regulation is possible for slight changes of the coating thickness, especially of the photographic light sensitive layer, this regulation system may be applicable to the control of the coated layer to minimize the thickness error correcting value which is required for obtaining photographic light sensitive materials having maximum density to a negligibly low level, thus enabling a saving of Ag consumption.

EXAMPLE 1 A coating apparatus equipped with a coating part having a mechanical vibration with a maximum amplitude of 2 and a frequency of 50-70 H and a multiple doctor type coating device was operated to coat two photographic industrial Rontgen emulsion layers on triacetylcellulose base web surface with a web speed of 40 m./ min. and an emulsion coating thickness of cc./m. The mechanical vibration was detected by a capacitor type vibration meter, and a signal voltage was obtained by passing the detected signal from the vibration meter through a polarity converter composed of semiconductor circuits, a phase shifter and a voltage amplifier composed of electron tubes.

This signal voltage had the maximum value of 500 v. and a suitable phase deviation proportional to the shape of the vibration wave was then applied between the multiple doctor type coating device and the backing roller together with positive bias voltage of 500 v. By this application of a signal voltage, the change of the coating thickness along the web running direction, otherwise shown as an irregularity having 9-13 mm. pitch, was extinguished.

EXAMPLE 2 A triacetylcellulose base web having the longitudinal thickness irregularity with a pitch of 15-30 mm. and the maximum thickness deviation of 6a Was coated with a four-layer photographic cinecolor positive emulsion by a multiple doctor type coating device with a web speed of 30 m./min. and a coating thickness of 200 cc./m. The change of the web thickness was detected by a contact by a contact type electron micrometer and the obtained signal was passed through a polarity converter composed of semiconductor circuits and a delay circuit and amplified by a voltage amplifier composed of semiconductor circuits to a signal voltage having a maximum value of 300 v. and the proper time lag and proportional to the shape of the thickness change wave. The resultant signal voltage and a 400 v. positive bias voltage were applied between the multiple doctor type coating device and the backing roller. By the application of the signal voltage, the change of the coating thickness along running direction of the web, otherwise exhibited as an irregularity having the same pitch, as in the Example 1, was extinguished.

We claim:

1. A method for controlling the thickness of a coating on a traveling backing web to compensate for variable parameters which adversely affect the thickness of the coating comprising measuring said variable parameters prior to the coating of the backing web, converting said measurements into an electric signal, varying the polarity and phase of said signal, amplifying said signal and applying the resultant electric signal as a potential difference between the coating composition and an electrode on the opposite side of said backing web during application of said composition to said backing web to vary the coating of the composition solely in response to said signal resulting from said parameters measured prior to the coating of the backing web to precisely control the thickness of the applied coating on the web.

2. A method as set forth in claim 1 wherein said step of measuring said variable parameters comprises measuring the vibration of the coating apparatus.

3. A method as set forth in claim 1 wherein the step of measuring said variable parameters comprises measuring the web thickness change prior to application of the coating material.

4. A method as set forth in claim 1 wherein the step of measuring said variable parameters comprises measuring irregularities in the speed of said traveling backing web.

7 c8 5. A method as set forth in claim 1' wherein a plurality 3,378,676 1 4/1968 Clement 1188 of Variable parameters are measured simultaneously and 3,462,286 "8/1969 DeGeest et a1 1l734 converted into a plurality of electric signals Which are 3,549,406 12/1970 Ambusk 11734 combined into a single mixed signal subsequent to the 1 I Y 1 l step of varying the phase and pclarity of said signals. 5 ALFRED V T, PflmaTY Exammer References Cied M ?A E UNITED STATES PATENTS US. Cl. X.R.

2,959,559 9/196o' Nadeau "1'17 93.4 R 10 11'7, 93.4"Nc,-13sis RQDIG. 2'; i1s' 7, s, 411, 629; 3,335,026 8/1967 De Geestetal. 117 93.4 R 235-151 s 

